Scalable and sustainable process of spike spherical Mg(OH)2 adsorbent from magnesite by ammonia-cycle method for dye removal

This study presents an innovative, and environmentally friendly synthesis process for spike spherical magnesium hydroxide (SSMH) using magnesite and an ammonia-cycle method, which eliminates waste liquid, gas, and chemical reagent pollution. The process involves calcining magnesite to obtain calcined magnesite, which reacts with (NH4)2SO4 to produce magnesium sulfate and ammonia. Subsequently, magnesium sulfate reacts with ammonia water to generate SSMH. The optimized conditions for the extraction of magnesium ion (Mg²⁺) are as follows: (NH4)2SO4 concentration of 2.4 mol/L, 4 g of calcined magnesite powder, and a reaction time of 1.5 h, resulting in an extraction rate of Mg²⁺ of 93.4%. The optimized conditions for the precipitation of Mg² ⁺ are as follows: Mg² ⁺ concentration of 0.7-1.3 mol/L, NH₃H₂O/Mg² ⁺ molar ratio of 9, reaction temperature of 60°C, and reaction time of 1 h, with a precipitation rate of Mg²⁺ of about 85%. After five ammonia cycles, the precipitation rate of Mg² ⁺ stabilizes at 85%. Scanning Electron Microscopy (SEM) confirms the spike spherical structure of the SSMH, with uniform particle diameters and a particle size of 2 μm. Adsorption studies indicate a maximum adsorption capacity (Qm) of 150.4944 mg/g for Reactive Red X-3B (RRX) dye at 25°C, fitting well with the Langmuir and pseudo-second-order kinetic model. The adsorption is primarily chemisorption-driven. In conclusion, the ammonia-cycle method for SSMH from magnesite is an environmentally friendly and sustainable approach. SSMH with its high adsorption capacity and spike spherical structure, is effective for treating RRX aqueous solution and has potential for broader applications in removing heavy metals, persistent organic compounds, nitrogen, and phosphorus from various polluted sources.